A Radioactive Lens

Between the 1940s and 1970s, a number of camera manufacturers
designed lenses employing
thoriated glass
in one or more elements. Incorporating as much as 40%
thorium dioxide
(ThO2) in the glass mixture increases the
index of refraction
of the glass while maintaining low
dispersion.
Thoriated glass elements allowed lenses to deliver low
levels of aberration and distortion with relatively simple
and easy to manufacture designs.

As with everything in engineering, there are trade-offs.
Thorium
is a radioactive element; it has no stable isotopes. Natural
thorium consists of 99.98% thorium-232, which has a half-life of
1.4×1010 years. While this is a long half-life,
more than three times that of uranium-238, it is still
substantially radioactive and easily detected with a
Geiger-Müller counter.
Thorium decays by alpha emission into radium-228, which
continues to decay through the
thorium
series into various nuclides, eventually arriving
at stable lead-208.

Attached to my Leica M6 film camera above is a Leica
Summicron 50 mm f/2 lens which contains thoriated glass.
Its serial number, 1041925, indicates its
year of
manufacture as 1952. This lens was a screw mount
design, but can be used on more recent bayonet mount
Leica cameras with a simple adapter. Like many early
Leica lenses, it is collapsible: you can rotate the
front element and push the barrel back into the camera
body when not in use, making the camera more compact to
pack and carry. Although 66 years old at this writing,
the lens performs superbly, although not as well as current
Leica lenses which are, however, more than an order of magnitude
more expensive.

To measure the radiation emitted by this
thoriated glass lens I used a
QuartaRAD RADEX RD1706
Geiger-Müller counter and began by measuring the
background radiation
in my office.

This came in (averaged over several measurement periods) as
0.12 microsieverts (μSv)
per hour, what I typically see. Background radiation
varies slightly over the day
(I know not why), and this was near the low point of the
cycle.

I then placed the detector directly before the front
element of the lens, still mounted on the camera. The
RADEX RD1706 has two Geiger tubes, one on each side of the
meter. I positioned the meter so its left tube would be as
close as possible to the front element.

After allowing the reading to stabilise and time average, I
measured radiation flux around 1.14 μSv/h, nearly ten
times background radiation. Many lenses using thoriated glass
employed it only for the front element(s), with regular
crown
or flint glass
at the rear. This limits radiation which
might, over time, fog the film in the camera. With such
lenses, you can easily detect the radiation from the front
element, but little is emitted backward in the direction
of the film (and the photographer). This is not the
case with this lens, however. I removed the lens from
the camera, collapsed it so the back element would be
closer to the detector (about as far as the front element
was in the previous measurement) and repeated the
test.

This time I saw 1.51 μSv/h, more than twelve times
background radiation. What were they thinking? First of all
the most commonly used films in the early 1950s were
slower (less sensitive) than modern emulsions, and
consequently less prone to fogging due to radiation. Second,
all Leica rangefinder cameras use a
focal-plane
shutter, which means the film behind the lens
is shielded from the radiation it emits except for the
instant the shutter is open when making an exposure,
which would produce negligible fogging. Since the
decay chain of thorium consists exclusively of alpha
and beta particle emission, neither of which is
very penetrating, the closed shutter protects the
film from the radiation from the rear of the lens.

Many camera manufacturers used thoriated lenses. Kodak
even used thoriated glass in its top of the line
800 series Instamatic cameras, and Kodak
Aero-Ektar lenses, manufactured in great quantity
during World War II for aerial reconnaissance,
are famously radioactive. After 1970, thoriated
glass ceased to be used in optics, both out of
concern over radiation, but also due to a phenomenon
which caused the thoriated glass to deteriorate over
time. Decaying thorium atoms create defects
in the glass called
F-centres
which, as they accumulated, would cause the glass
to acquire a yellowish or brownish tint. This
wasn't much of a problem with black and white film,
but it would cause a shift in the colour balance
which was particularly serious for the colour reversal
(transparency) film favoured by professional photographers
in many markets. (My 1952 vintage lens has a slight
uniform yellow cast to it—much lighter than a
yellow filter. It's easy to correct for in digital
image post-processing.) Annealing the glass by
exposing it to intense ultraviolet light (I've heard
that several days in direct sunlight will do the job)
can reduce or eliminate the yellowing.

Thorium glass was replaced by glass containing
lanthanum oxide
(La2O3), which has similar optical properties.
Amusingly, lanthanum is itself very slightly radioactive: while the
most common isotope, lanthanum-139, which makes up 99.911% of
natural lanthanum, is stable, 0.089% is the lanthanum-138
isotope, which has a half-life of 1011 years, about
ten times that of thorium. Given the tiny fraction of the
radioisotope and its long half-life, the radiation from
lanthanum glass (about 1/10000 that of thorium glass), while
detectable with a sensitive counter, is negligible compared to
background radiation.

If you have one of these lenses, should you be worried?
In a word, no. The radiation from the lens is absorbed
by the air, so that just a few centimetres away you'll
measure nothing much above background radiation. To receive
a significant dose of radiation, you'd have to hold the front
element of the lens up against your skin for an extended period
of time, and why would you do that? Even if you did, the
predominantly alpha radiation is blocked by human skin, and the
dose you received on that small patch of skin would be no more
than you receive on your whole body for an extended period on an
airline flight due to cosmic rays. The
only danger from thorium glass would be if you had a telescope or
microscope eyepiece containing it, and looked through it with
the naked eye. Alpha radiation can damage the cornea of the
eye. Fortunately, most manufacturers were wise enough to avoid
thoriated glass for such applications, and radioactive eyepieces
are very rare. (Still, if you buy a vintage telescope or
microscope, you might want to test the eyepieces, especially
if the glass appears yellowed.)